1. Technical Field
The embodiments herein generally relate to wireless communication systems, and, more particularly, to downconversion in radio frequency (RF) receivers.
2. Description of the Related Art
In wireless communication systems, automatic gain control (AGC) is used to adjust the gain of the receiver to overcome the large variation in the received signal level. The variable gain low noise amplifier (LNA) and variable gain amplifier (VGA) could be designed in two different ways. The first is to have a linear gain response. In other words, the LNA and VGA should be able to have a gain that is directly proportional to the control voltage with the same proportion for all values of the input voltage. The second is to have a stepped gain response. In other words, the LNA and VGA have only a few discrete values for the gain. Switching from one gain value to another depends on the value of the control voltage. A stepped gain response leads to a much simpler implementation and a much lower power consumption for both the LNA and VGA. However, stepped gain response could cause significant glitches and a rather large gain discontinuity to the received signal, which generally does not happen with a linear gain response.
In orthogonal-frequency-division-multiplexing (OFDM) systems, a stepped gain response generally cannot be tolerated as any glitch or sudden change in the signal level may cause the loss of an entire OFDM symbol. This could lead to a significant loss of data, especially in the case of long OFDM symbols that are normally used in terrestrial television (TV) applications. In such applications, the length of one OFDM symbol could be as long as one millisecond. Accordingly, stepping the gain suddenly within any portion of an OFDM symbol could lead to the loss of the entire symbol. Therefore, there remains a need for a technique for enabling the use of stepped gain amplifiers for OFDM systems without any loss in performance.